This project will involve an investigation of the uterine and placental factors which have effects on ovo-implantation and maintenance of pregnancy in rabbits. We intend to characterize the cytologic response of the uterine epithelium to steroid hormones (progestins, estrogen). By modifying the hormonal environment and employing blastocyst substitutes we hope to define the parameters of uterine receptivity to blastocyst stimulation. Likewise, the endometrial response to such stimuli is to be studied. This will include an analysis of endometrial luteolytic activity and the mechanism by which the maternal placenta (decidua) overrides this luteolytic influence.